Panels for large industrial dryers conventionally comprise two sheets of metal forming the inside high temperature surface and an outer surface exposed to ambient temperature and separated from the inside sheet by packed insulation such as glass or mineral fiber or foam, typically 1 to 10 inches in thickness. The panel ends or edges, which make up the remainder of the panel structure, and which serve to hold the inner and outer sheets together and retain the insulation in place, are often also of sheet metal and may include means for accommodating a sealing gasket, such as an asbestos or silicone rubber gasket, which bears against a structural frame member or element of the dryer. Such panels may be designed for clamping to upright frame elements, or may be hinged to act as access doors.
Heat transfer through the main body of a panel is effectively impeded by the insulation layer. However, the path through the sheet metal panel ends or edges offers a much lower thermal resistance than the insulation, resulting in heat leakage from the inside of the dryer enclosure to the lower temperature surface on the outside of the panel near the panel ends or edges. Often the heat leakage is significant enough to constitute a serious safety hazard.
A well-established technique for minimizing this problem is the use of non-metallic inserts, typically asbestos cement board in some configuration, in the structure of the panel ends or edges to act as barriers to conductive heat transfer through such ends or edges. Examples are shown in prior U.S. Pat. Nos. 2,912,725 and 3,991,242. These inserts are effective but significantly increase the cost of the panel and sometimes its durability due to special manufacturing techniques required for working with the insert material.